This invention is related to wireless data network receivers, and in particular to a method and apparatus for determining the initial time reference when receiving a packet in a packet based wireless network such as a wireless local area network (WLAN).
It is particularly applicable to a receiver that conforms to any of the OFDM variants of the IEEE802.11 standard such as IEEE 802.11a and 802.11g.
A station of a wireless network includes a physical layer processor (PHY) that includes radio receiver and radio transmitter, i.e., a radio transceiver, and a MAC processor. The receiver of the PHY needs to accurately detect the start of packet (SOP), and once the SOP is detected, to accurately determine a time reference for the received signals. In an OFDM receiver wherein received data is subject to a discrete Fourier Transform (DFT), usually as an FFT, the time reference is used to determine when to start the first DFT.
It in known to use the received signal strength indicator (RSSI) for so detecting the SOP. The resulting determined SOP time is in general not sufficiently accurate to provide initial timing estimate. For example, for the IEEE 802.11b standard, the time for the packet energy to rise from 10-to-90% may be as wide as 2 μs. We have found that this is too wide for accurate initial timing determination. Furthermore, by carrying our experiments, we found that there is more than one transient present, and that there is a very limited window to accurately determine when the RSSI jumped most quickly.
Packets for communication in a wireless network typically include a preamble and a modulated data part. The preamble provides for start of packet detection, automatic gain calculation, synchronization, and so forth, and have known parts. Correlation based methods also are known that detect known parts of the preamble in each packet. An IEEE 802.11a preamble, for example, includes ten known short symbols and two known long symbols with guard intervals. We have used such correlation-based techniques in an IEEE 802.11a radio chip and found that there were still missed packets. For example, one method we have tried used correlating the preamble with the known guard interval for the long symbol that occurs after the ten short symbols. A correlation threshold was set to determine the time. If we set the correlation threshold to be sufficiently high to avoid false triggers off the short symbols, and there was significant multipath, we found that such a method sometimes produce false negatives (missed packets). We believe this is because the multipath may reduce the size of correlation peak for the correlation with guard interval.
Thus there is a need in the art for an accurate initial timing estimation.